1. Field of the Invention
This invention relates to active optical systems and more particularly to an active optical system for phase-shifting desired portions of an incoming optical wavefront.
2. Description of the Related Art
Many types of active optical systems require the control of the optical wavefront or phase of a propagating laser beam. When an image propagates through turbid media, for example, the atmosphere, random fluctuations in the local index of refraction cause local fluctuations in the optical path length that the beam traverses. These fluctuations in path length lead to a randomization of the phase front contour, causing the image to be obscured. Using an adaptive optics or active optical control, the original phase state is restored, allowing the reconstruction of the original image. In the case of optical communications, the same kind of randomization can occur. In this case, the adverse result is that the optical beam cannot be focussed to a diffraction limited (limited by wavelength) spot, causing loss of information when the beam is introduced into a small diameter optical element, for example, an optical fiber. Active control and adaptive optics in this scenario allows one to reconstruct the original phase state so that the beam can be focussed to a small spot without loss of information. Typically, active optical systems make use of adaptive optical elements that are based on mechanical implementation. One example of this is a deformable mirror. The mirror contains a number of small actuators that push or pull on the mirror surface. In doing so, they compensate for the distortions in the beam phase by making some parts of the optical path shorter and some parts of the optical path longer. However, this implementation takes what is fundamentally an optical problem and turns it into a mechanical problem. It is desirable to use a non-mechanical system to accomplish the phase-shifting needed to recreate the original phase state of the optical beam.
There have been previous patents to use electro-optical means to perform adaptive optical processes. U.S. Pat. No. 5,396,364, entitled CONTINUOUSLY OPERATED SPATIAL LIGHT MODULATOR APPARATUS AND METHOD FOR ADAPTIVE OPTICS, issued to O""Meara et. al, discusses the use of a spatial light modulator for electro-optically addressed adaptive optics. A standard SLM is described, that incorporates an electronically xe2x80x9cpixelatedxe2x80x9d modulator. The device incorporates a microlenslet array to physically separate the wavefront into small active areas that form the pixels. This device has several disadvantages. The electronic structure must be built directly into the device, causing greater difficulty in manufacture and limiting the resolution of the device to the number of electronic structures created.
U.S. Pat. No. 6,222,667, entitled ELECTRO-OPTIC LIGHT VALVE ARRAY, issued to Gobeli et, discloses a two-dimensional light valve array. It uses a pixelated substrate made of lanthanum modified zirconate-titanate. Electrodes are cut into recesses made in the substrate. Voltages which are applied to the individual pixels induce bi-refringence into the pixelated regions. Electronic control of the bi-refringence affects the light transmittance. The inventor does not discuss control of phase or wavefront in this device. The lanthanum zirconate-titanate material is expensive and difficult to manufacture. While its electro-optical response is reasonable for the application, small inconsistencies will cause deterioration in its performance.
The present invention is an active optical system and method for phase-shifting desired portions of an incoming optical wavefront. A control optics assembly receives an incoming optical wavefront and adjusts that incoming optical wavefront in accordance with first desired wavelength and beam propagation parameters. The spatial light modulator (SLM) addressing optics receives the control optics output wavefront and produces a desired beam size therefrom. An SLM receives the output from the SLM addressing optics and provides localized phased shifting of the resulting wavefront. The SLM comprises a microscopic array of pixelated silicon nanocluster elements and a voltage source for applying independently controlled voltages on desired nanocluster elements. SLM egressing optics receives the output of the SLM and returns the beam size of the wavefront to the initial beam size. The output of the SLM egressing element has desired portions of its phase shifted relative to the incoming optical wavefront.
The present invention performs phase control on an optical wavefront without utilizing a deformable mirror to compensate for phase distortions produced by atmospheric conditions. By altering the manner in which the imaging device is addressed, the local refractive index of the two-dimensional medium can be used to modulate or demodulate the wavefront at a single position within the wavefront. This results in a phase compensated wavefront.